The Gram-negative bacterium, Neisseria gonorrhoeae, is the only causative agent of the sexually transmitted disease gonorrhea. One important facet of gonococcal pathogenesis is the process of pilin antigenic variation (Av), which allows for immune evasion by the bacterium and continual retransmission through the high risk/core group of infected people. In addition, the pilus functions in the initiating events of colonization and the functional properties of the pilus can be altered by pilin antigenic variation. Understanding the mechanisms used to effect and control pilin Av is essential to understanding this aspect of the pathogenesis of gonococcal disease. This proposal will continue our genetic studies into the molecular mechanisms used to enable pilin Av. There are two specific aims: Aim 1 will continue our identification and characterization of genes and protein products, revealed by a genetic screen, that are involved in mediating and regulating pilin antigenic variation. Aim 2 will explore the mechanisms of pilin antigenic variation by defining the role of conserved DNA sequences in pilin antigenic variation, by testing novel models that explain pilin antigenic variation, by defining how variability of pilin variation is maintained, and by re-examining a controversial hypothesis that DNA transformation has a role in mediating pilin Av. The results from these studies will help maintain pilin antigenic variation as one of the main prokaryotic paradigms for systems based on genomic translocations, and further our understanding of how DNA recombination can be central to an important mechanism of microbial pathogenesis. [unreadable] [unreadable]